Self-biased development electrode and reproducing machine employing same

ABSTRACT

A development apparatus for developing electrostatic charge patterns or images or a dielectric surface which includes an electrically biased development electrode in close proximity to the surface. The electrode is connected to ground potential through an impedance path which comprises at least one reversed biased zener diode. The electrode is self-biased with the biasing potential being a function of the breakdown voltage of the zener diode. The breakdown voltage of the diode is selected to give the desired electrode potential. Preferably, the apparatus includes means for changing the bias potential on the electrode. An electrostatographic reproducing apparatus employing the aforenoted developer apparatus is also claimed.

United States Patent 191 North et al.

[111 3,889,637 June 17,1975

ELECTRODE AND REPRODUCING MACHINE EMPLOYING SAME [75] Inventors: Arthur J. North, Rochester;

Raymond W. Stover, Webster, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: June 28, 1973 [21] Appl. No.2 374,295

[52] US. Cl 118/637; 117/17.5 [51] Int. Cl G03g 13/06 [58] Field of Search 118/637; 117/175; 355/3 [56] References Cited UNITED STATES PATENTS 3,257,223 6/1966 King 118/637 3,328,193 6/1967 Oliphant et al.... 118/637 3,599,605 8/1971 Ralston et a1 118/637 3,654,893 4/1972 Piper et a1. 118/2 SELF-BIASED DEVELOPMENT Morse 118/637 Trachienberget a1 118/7 [57] ABSTRACT A development apparatus for developing electrostatic charge patterns or images or a dielectric surface which includes an electrically biased development electrode in close proximity to the surface. The electrode is connected to ground potential through an impedance path which comprises at least one reversed biased zener diode. The electrode is self-biased with the biasing potential being a function of the breakdown voltage of the zener diode. The breakdown voltage of the diode v is selected to give the desired electrode potential. Preferably, the apparatus includes means for changing the bias potential on the electrode. An electrostatographic reproducing apparatus employing the aforenoted developer apparatus is also claimed.

20 Claims, 3 Drawing Figures SELF-BIASED DEVELOPMENT ELECTRODE AND REPRODUCING MACHINE EMPLOYING SAME BACKGROUND OF THE INVENTION This invention relates to an apparatus for developing an electrostatic charge pattern or image on a dielectric surface wherein a self-biasing development electrode is employed and to an electrostatographic reproducing machine incorporating the developing apparatus.

Perhaps the best known electrostatographic process comprises xerography. This process is very well known in the art and is the subject of numerous patents and texts, including Electrophotography by Schaffert, published in 1965, and Xerography and Related Processes, by Dessauer and Clark, published in 1965. The basic process is also set forth in US. Pat. Nos. 2,221,776 and 2,357,809.

A large number of ways have been employed to develop the electrostatic charge pattern on a dielectric surface, including cascade development, powder cloud development, magnetic brush development, fur brush development, liquid development and other techniques as are well known in the art. A problem has existed in developing central portions of large solid image areas due to the absence of strong fields at the central portions. In order to overcome this problem, the art has employed a device known as a development electrode, which is held close to the surface of the plate containing the electrostatic image. The development electrode may be connected directly to ground potential or it may be electrically biased by connection to a suitable voltage source such as a power supply or it may be electrically self-biased by connection to ground potential through a high impedance.

The effect of the electrode is to change the field configuration of the electrostatic image and to increase the field in the space above the large solid areas of charge. Under these conditions the various parts of the electrostatic image are developed out more nearly in proportion to the electrical charge density on the plate surface. However, a problem associated with the use of a development electrode is that is promotes the development of background areas. These areas which are usually at a low potential with respect to the image areas carry sufficient residual charge to attract toner particles. Where the electrode is connected to an external power source the bias on the electrode can be maintained at an electrical potential greater in magnitude than the background area so as to inhibit or eliminate background development. This bias potential is, however, less than the potential on the image areas so that development of the image areas are not substantially suppressed.

In addition, it has been found desirable by the art to provide means for varying the bias on the electrode in order to change the density of the developed image. For example, where the original does not provide sufficient contrast to obtain a good image under normal developing conditions, the bias on the electrode may be lowered to increase the density of the image with some increase in background development. This is commonly known as a light original mode of development.

Grounded electrodes do not provide for background suppression since the electrode is not biased at a potential above the background level. The provision of biased electrodes using an external power source is generally expensive and utilizes relatively complex circuitry.

A means for self-biasing the electrode to suppress background development is set forth in US. Pat. No. 3,599,605. This patent discloses an apparatus employing a development electrode for developing a latent electrostatic image on a charged number. Specifically, the patent discloses grounding the electrode through an electrical impedance circuit, generally comprising a high valued resistor or a resistor and capacitor in parallel. The patent suggests that a potential is provided at the electrode which results from the absorption of a portion of the charge induced on the electrode by the image. The resistor constitutes an impedance which retards the flow of charge from the electrode to ground and has the effect of maintaining the electrode at a potential above ground during the time that an electrostatic image is being developed. The value of the resistance is chosen to provide the potential necessary to give the desired background suppression.

The apparatus set forth in this patent is subject to wide variations in bias voltage due to the variation in the ratio of charged areas to discharged areas. The patent discloses voltage variations of about 200 volts. By changing the impedance from a resistor to a resistor and capacitor in parallel, the voltage variations were dampened to about volts. While a 100 volt variation in bias potential could be tolerated for background suppression if the overall bias potential is set sufficiently higher than the background potential, it would make it extremely difficult to provide a uniform consistent light original mode of operation since in this latter mode the potential on the electrode is dropped by about the same order of magnitude as the voltage variation observed when utilizing the apparatus of the above-noted patent.

It has been found in accordance with this invention to be highly desirable to regulate the bias potential on the electrode. A stabilized bias potential allows one to change the level of the bias potential to obtain desired increases or decreases in the density of the developed image.

In US. Pat. No. 3,674,532 there is disclosed a selfbiasing electrode apparatus wherein the electrode is grounded through an electrical resistance as in accordance with the previously noted patent. In accordance with this last mentioned patent, the bias potential is thought to be comprised of an induced voltage component and a triboelectrically derived component. A further circuit is provided for preventing downward fluctuations away from an arbitrary voltage of the bias potential from exceeding a pre-determined magnitude. The arbitrary voltage most often used is derived from the surface potential of the element being developed. The apparatus senses changes in the surface potential of the element, compares the potential so sensed with a stored potential, and supplies an additive corrective potential when needed. While this patent recognizes the need for regulating the bias potential on the development electrode, it accomplishes this regulation by a very complex circuit.

SUMMARY OF THE INVENTION In accordance with this invention a development apparatus for developing electrostatic charge patterns or images on a dielectric surface is provided which includes an electrically biased development electrode in close proximity to the surface. The electrode is connected to ground potential through an impedance path which comprises at least one reverse biased zener diode. The charge flow from the development electrode produced during development of the charge pattern is applied to the zener diode and provides the self-biasing potential for the electrode. The bias potential on the electrode is therefore a function of the breakdown voltage of the zener diode. A zener diode provides a stable breakdown voltage over a wide range of current values. The use of a zener diode in accordance with this invention represents a unique and simple apparatus for obtaining a self-biasing development electrode wherein the bias potential is regulated or stabilized irrespective of the charge transients which result from differences in the charge pattern.

The breakdown voltage of the diode is selected to give the desired electrode potential. The apparatus preferably includes means for changing the bias potential on the electrode. The electrode potential can be varied, for example, by providing a plurality of diodes of the same or differing values of breakdown potential and a switching means for selecting between one or more of the diodes to provide different cumulative breakdown potentials and concomitant electrode potentials. Preferably the zener diodes employed are stable within about i percent of their breakdown voltages at a current of 0.1 microamps.

The apparatus in accordance with this invention is well suited to applications requiring the selection of a plurality of discrete bias potentials to provide images of differing densities. The uniquely stabilized nature of the bias potential provided herein allows uniform and consistent results to be obtained at these discrete bias potentials.

An electrostatographic reproducing machine employing the developing apparatus as aforenoted also forms part of this invention.

Accordingly, it is an object of this invention to provide an improved developing apparatus for developing electrostatic charge patterns on a dielectric surface.

It is a further object of this invention to provide an apparatus as above including an electrically self-biased development electrode.

It is a further object of this invention to provide an apparatus as above wherein the bias potential of said electrode is stabilized at a given potential.

It is a further object of this invention to provide an apparatus as above further including means to change the bias potential on the electrode.

It is a further object of this invention to provide an electrostatographic reproducing machine incorporating the improved developing apparatus set forth above. These and other objects will become more apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows schematically an electrostatographic reproducing machine in accordance with this invention including the developing apparatus of this invention.

FIG. 2 shows a partial cross-section of an exemplary developing apparatus of the type set forth in FIG. 1.

FIG. 3 shows a schematic diagram of an exemplary circuit for obtaining various discrete bias potentials on the developing electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown by way of example an electrostatographic reproducing machine I incorporating a developing apparatus 2 in accordance with this invention. A xerographic drum 10 is arranged to operatively communicate with a circulating toner carrying magnetic brush development system 2. The specific apparatus shown is a xerographic copier, however, the apparatus of this invention is applicable to any desired electrostagographic system, including as for example, xeroradiography, xeroprinting, electrographic recording. The practice of xerography is well known in the art as set forth in the texts and patents previously noted. The various processing stations for making a xerographic copy from an original are herein represented as blocks A through E. The xerographic drum 10 comprises a conductive substrate, the outer periphery of which is coated with a suitable photoconductive material 11. One type of suitable photoconductive material is disclosed in US. Pat. No. 2,970,906 issued to Bixby in 1961. The drum 10 is suitably journaled for rotation within a machine frame (not shown) by means of a shaft 12 and rotates in the direction indicated by arrow 13, to bring the image retaining surface thereon past a plurality of xerographic processing stations. Suitable drive means (M) are provided to power and coordinate the motion of the various cooperating machine components.

Initially the drum 10 moves photoconductive surface 11 through charging station A. In charging station A an electrostatic charge is placed uniformly over the photoconductive surface 11 of the drum l0 preparatory to imaging. The charging may be provided by corona generating device of a type described in US. Pat. No. 2,836,725 issued to Vyverberg in 1958.

Thereafter, the drum I0 is rotated to exposure station B where the charged photoconductive surface 11 is exposed to a light image of the original input scene information, whereby the charge is selectively dissipated in the light exposed regions to record the original input scene in the form pf a latent electrostatic image. After exposure, drum 10 rotates the electrostatic latent image recorded on the photoconductive surface 11 to development station C wherein a conventional developer mix is applied to the photoconductive surface 11 of the drum l0 rendering the latent image visible. A suitable development station is disclosed in US. Pat. No. 3,707,947 issued to Reichart in 1973. The magnetic brush development system described utilizes a magnitizable developer mix having carrier granules and a toner colorent. The developer mix is continuously brought through a directional flux field to form a brush thereof. The electrostatic latent image recorded on photoconductive surface 11 is developed by bringing the brush of developer mix into contact therewith.

The developed image on the photoconductive surface II is then brought into contact with a sheet 14 of final support material within a transfer station D and the toner image is transferred from the photoconductive surface Ill to the contacting side of the final support sheet 14-. The final support material may be paper, plastic, etc., as desired. After the toner image has been transferred to the sheet of final support material 14, the sheet with the image thereon is advanced to a suitable fuser 15 which coalesces the transferred powder image thereto. One type of suitable fuser is described in U.S. Pat. No. 2,701,765 issued to Codichini, et al in 1955. Although a preponderance of the toner powder is transferred to the final support material 14 invariably some residual toner remains on the photoconductive surface 11 after the transfer of toner powder image to the final support material 14. The residual toner particles remaining on the photoconductive surface 11 after the transfer operation are removed from the drum as it moves through cleaning station E. Here the residual toner particles are first brought under the influence of a cleaning corona generating device (not shown) adapted to neutralize the electrostatic charge remaining on the toner particles. The neutralized toner particles are then mechanically cleaned from the photoconductive surface 11 by conventional means as, for example, the use of a resiliently biased knife blade as set forth in U.S. Pat. No. 3,660,863 issued to Gerbasi in 1972.

It is believed that the foregoing description is sufficient for purposes of the present application to illustrate the general operation of an automatic xerographic copier 1 which can embody the teachings of the present invention.

Referring now more specifically to FIGS. 1 and 2, the developing apparatus 2 of the present invention will be more fully described. The developing apparatus 2 includes an electrically isolated magnetic brush which is contained within a housing 21. The magnetic brush 20 can operatively communicate with the latent electrostatic image or charge pattern carried on the photoconductive surface. The developing apparatus 2 shown in FIG. 2 comprises a single magnetic brush forming unit 1, and a magnetic developer unit 22 which is positioned directly above and in parallel alignment with the brush forming unit 20. Developer material within the sump region 24 of the housing 21 is initially picked up by the brush forming unit 20 and passed through the active development zone and eventually transferred to the lifting unit 22 which transports the material to the top of the unit 22. Here the developer material is released from the lifting unit 22 and deposited upon an inclined chute 25 where, under the influence of gravity, the material is allowed to move down the chute where it ultimately falls into a series of cross-mixing baffles 26. The cross-mixing baffles 26 are better illustrated in U.S. Pat. No. 3,707,947. Upon leaving the cross-mixing baffles the now properly charged and uniformly distributed developer material is allowed to fall back into the sump 24 where it once again can be used in the developement process.

Note the magnetic brush forming unit 20 and the magnetic developer lifting unit 22 are generally made up of outer applicator roll members 27 and 28 which are arranged to substantially enclose magnetic flux generating devices 29 and 30. The roll member 27 and 28 are suitably driven at a pre-determined rate.

In operation this roll member 27 associated with the magnetic brush forming unit 20 is driven sequentially through the developer sump 24 and the active development zone 31. The material is then carried on the surface of the rotating member 27 into the main flux field of the brush forming magnet 29 wherein the developer particles are caused to align themselves along the main lines of flux to form a brush-like structure. Because of the magnet 29 arrangement the brush fibers extend outwardly through the developer housing 21 into operative communication with the photoconductive surface 11 of the drum 10. As the drum rotates in the direction indicated, the brush fibers are caused to be drawn over the photoconductive surface 11 as it is moved through the development zone 31. As a consequence, toner particles are electrically transferred from the brush fibers to the image region on the photoconductive surface 11, thereby rendering the image as visible. The developer material continues to move through the active development zone 31 and is brought into close proximity with the lifting unit 22. Here, the developer material moves across the magnetic bridge linking the two units 20 and 22 and is secured against the roll member 28 which in turn deposits it upon the inclined chute 25. The chute directs the developer through the cross-mixing baffle 26 from which it is subsequently returned to the developer sump region 24.

By suitably electrically biasing the magnetic brush roll member 27, and thereby the brush itself, the developability of the system is improved. The brush 32 thereby acts as a development electrode.

Conventionally the development electrode in any given development system is placed within the active development zone in close proximity to the dielectric surface such as the photoconductive surface 11 and an electrical bias having a potential generally higher, but close to the background potential recorded on the plate surface 11 is applied thereto. Background in a term of art used to define those regions of the plate surface 11 which although carrying a weak potential, nevertheless do not contain the desired charge pattern or electrostatic image. Although the background regions are reduced to a relatively low charge potential during the xerographic exposure process, they can under certain conditions, attract and hold random toner particles that are brought into contact therewith. By biasing the development electrode 32 to a potential somewhat greater than the background potential found on the plate surface 11, an electrical force field is established within the development zone which tends to pull the toner away from the plate when a background region is moved therethrough thereby suppressing background development.

Referring again to U.S. Pat. Nos. 3,559,605 and 3,674,532, there are disclosed apparatuses for providing self-biasing development electrodes. The apparatuses provide for connecting the electrodes to a ground potential through a high impedance path. The deficiencies of the apparatuses of these patents have been set forth in the background of this invention.

Referring now to FIG. 1 in accordance with this invention a unique, simple and inexpensive means for self-biasing the development electrode has been provided which comprises connecting the development electrode 32 to a ground potential through a reverse biased zener diode 40. The zener diode 40 is reverse biased sufficiently to exceed its breakdown potential so that it regulates or stabilizes substantially at that potential. The potential generated across the zener diode 40 comprises the potential which is applied to the development electrode. The current which generates the selfbiasing potential across the diode 40 is thought to be made up of several components. One component comprises an induced current resulting when the development electrode 32 is brought into close proximity with the charged image on the photoconductor surface 11 due to an induced charge appearing on the surface of the magnetic brush. Since there is an impedance between the brush 32 and the reference voltage which in this embodiment is ground potential, charges flow between the brush and the source of reference voltage and a voltage appears across the impedance element which comprises the bias potential. The greater the charge density on the image or layer 1 l, the greater the induced charge, thus, producing a greater current flow through the impedance element.

Other contributions of the bias potential result from a triboelectric current component. When the conductive magnetic brush 31 is brought into close proximity with the charge pattern on the surface layer 11 of the drum l0, toner is transferred from the brush 32 to the surface 11. There then remains on the brush 32 an excess of charge having a polarity opposite to that of the lost toner particles. Since there is an impedance element 40 between the magnetic brush and the reference voltage or ground potential, a current flows between the brush 32 and the reference source and a voltage appears across the impedance element 40 as a bias potential. The greater the rate at which toner is removed from the brush 32, the greater is the current flowing across the diode 40.

In accordance with this invention a reverse biased zener diode 40 acts as a voltage regulator and makes use of the constant voltage characteristic of the diode. The zener diode 40 breakdown region provides for a large change in reverse current over a very narrow range of reverse voltage. This characteristic permits a highly stable voltage to be maintained across the diode 40 dispite a relatively wide range of currenfthrough the diode. The voltage generated across the diode 40 comprises the potential which is applied to the development electrode 32. Therefore, the development electrode 32 potential is set by providing a single zener diode 40 or a plurality of diodes 40 selectively connected between the electrode 32 and the ground potential to establish and stabilize the bias potential on the electrode.

The breakdown potential for the zener diode 40 or diode combination employed may be set as desired to achieve the desired potential on the development electrode 32. it is preferred in accordance with this invention that the breakdown potential for the diode 40 or combination diodes for normal machine operation be at least 25 and more preferably at least 50 volts above the background potential in magnitude on the dielectric surface 11. For a light original mode of operation the breakdown potential on the diode 40 or diode combination preferably should be less than about 100 volts below the background potential in magnitude on the dielectric surface 11 and more preferably less than about 50 volts below that potential.

The electrode potential may be generated in accordance with this invention across a single zener diode 40 or a plurality of diodes selectively connected as shown at 41. Series connections 41 provide relatively easy means for increasing the total potential applied to the development electrode 32. Therefore, for example, two zener diodes 42 and 43 having about 100 volt breakdown potentials connected in series would generate an electrode potential of about 200 volts.

The zener diodes 40 in accordance with this invention are capable of operating with a development electrode current of at least 0.1 microamperes and preferably at least 0.01. Preferably in accordance with this invention the regulating voltage of the zener diodes 40 is within about plus or minus l0 percent of the nominal breakdown voltage at 0.1 microamps and more preferably within these limits at 0.01 microamps.

Referring to FIG. 3 there is shown an exemplary means for varying the potential supplied to the electrode 31 which comprises a circuit 50 employing a plurality of zener diodes 40 arranged to provide a plurality of development electrode potentials. The circuit 50 comprises a number of zener diodes 40 and 4246 having different nominal breakdown potentials and a switching means 51 for selectively connecting one or more of said diodes 40 and 42-46 between the develop ment electrode 31 and ground potential. The circuit 50 shown is but one means for switching selectively between zener diodes 40 or diode combinations 41 and various other circuits could be devised to accomplish the same function. What is desired is a means for providing different regulated electrode potentials so that, for example, one may provide differing degrees of image density merely by changing the setting of the switch 51. Another type of switching arrangement (not shown) which could be employed for example, would comprise providing a plurality of zener diodes 40 in series having the same value or mixed values of breakdown potential as desired and switching means for connecting to many of the diodes in series as desired to ob tain the desired electrode potential.

It is believed in accordance with this invention that the current generated at the development electrode 31 is a function of the copy width, the measured output optical density, the average toner charge in microcolumbs, the percent coverage of the document by toner and the processing speed. For most machines now in use these currents range from about 0.05 microamps up to about 200 microamps.

The invention will now be illustrated more fully by reference to a specific example.

EXAMPLE I The following tests were performed using a magnetic brush flat plate fixture with a single one inch roll. Prints were made alternating between a 94 volt zener diode biasing arrangement in accordance with this invention, and a 94 volt power supply biasing arrangement. Prints were also made with three diodes in series for a bias of 325 volts and an equivalent print with a 325 volt bias from the power supply. Only visual analyses of the prints were made.

Print quality differences between diode biasing and power supply biasing were next to nill. The tests showed that even a single roll development system could produce in excess of 325 volts during the development process using diode biasing.

These tests fully establish that zener diode biasing can be employed in place of conventional power supply systems.

The zener diodes used in the aforenoted tests were manufactured by Motorola and may be purchased under the following designation: 1N4764. The copy width was 8 inches, the measured output optical density was about 1.3, the percent coverage was about 5 percent, and the processing speed was 6 inches per second. Under these conditions the current generated at the development electrode was measured to be about 0.5 microamperes.

The zener diode 40 or diode combination 41 may be connected to the development electrode by any conventional means.

Referring to FIGS. 1 and 2, the diode 40 is shown connected to the magnetic brush unit by means of a conductive wiper element 60 which electrically contacts the side wall or shaft (not shown) of the roll member 27.

While the invention has been described with reference to the use of dielectric or photoconductive materials as the electrostatic pattern or image bearing surface, any desired material or combination of materials capable of bearing the electrostatic charge pattern or image could be employed as the image bearing surface.

While the invention has been described with reference to generating the electrode potential across a zener diode connected between ground potential and the development electrode, it should be evident that the ground potential may alternatively comprise any desired pre-set reference voltage. The use of a reference voltage other than ground potential is not preferred in accordance with this invention, but could be employed in those applications where it is desired.

The term electrostatography as employed in the present application refers to the formation and utilization of electrostatic charge patterns for the purpose of recording and reproducing patterns in viewable form. Electrostatography specifically includes while not being limited to electrography wherein an insulating medium is employed to form without the aid of electromagnetic radiation latent electrostatic charge patterns for producing a viewable record and electrophotography which employs a photoresponsive medium to form with the aid of electromagnetic radiation, latent electrostatic charge patterns for producing a viewable record Reverse bias as the term is used herein refers to the bias created across the diode by the current from the electrode. The polarity of current from the electrode will depend, of course, on the particular electrostatographic system polarities.

Any current leakage paths to ground potential from the development electrode must have a magnitude less than the current flowing through the diode and preferably less than 0.1 microamps and more preferably less than 0.01 microamps.

While zener diodes are known to be useful as bias and coupling elements in vacuum tube and transistor circuitry for audio and RF applications as noted for example in the International Rectifier Corporation, Zener Diode Handbook published by International Rectifier Corporation, El Segundo, California in 1960, it is certainly surprising in the electrostatographic art that zener diodes can be employed as a biasing element for self-biasing development electrodes. The art as exemplified by U.S. Pat. No. 3,674,532 has gone to extraordinarily complicated circuitry in order to provide voltage regulation for a self-biased development electrode.

The patents and texts referred to specifically in detailed description of this application are intended to be incorporated by reference into the description.

It is apparent that there has been provided in accordance with this invention a self-biased development electrode apparatus and an electrostatographic reproducing machine incorporating the same which fully satisfies the objects, means and advantages set forth hereinbefore. While the invention has been described in conjunction with specific embodiments therefore, it is evident that many alternatives modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.

What is claimed is:

1. In a developing apparatus for developing electrostatic charge patterns on a surface, including an electrically biased development electrode in close proximity to said surface, said electrode being connected to a reference potential through an impedance means so that the charge produced on the electrode during the development of said pattern provides the bias potential for said electrode, the improvement wherein, said impedance means comprises: at least one zener diode connected between said electrode and said reference potential so that the charge produced on said electrode during the development of said pattern provides a reverse bias potential across said zener diode to cause it to stabilize the bias potential on said electrode at a desired level.

2. An apparatus as in claim 1 wherein said reference potential comprises ground potential and wherein said zener diode regulates within about plus or minus 10 percent of its nominal breakdown voltage at an electrode current of about 0.1 microampheres.

3. An apparatus as in claim 2 wherein said zener diode has a breakdown potential of less than about 50 volts above the background potential on said surface.

4. An apparatus as in claim 2 wherein said zener diode has a breakdown potential of less than about volts below the background potential on said surface.

5. An apparatus as in claim 2. wherein a plurality of zener diodes are selectively connected between said electrode and said ground potential to provide said bias potential.

6. An apparatus as in claim 5 wherein said zener diodes are connected in series.

7. An apparatus as in claim 2 further including means for changing the bias potential on said electrode.

8. An apparatus as in claim 7 wherein said potential changing means comprises a plurality of zener diodes or combinations of zener diodes wherein each diode or diode combination has a differing breakdown potential, and switching means for selectively connecting one or more of said diodes or diode combinations between said development electrode and said ground potential.

9. An apparatus as in claim 8 wherein said developing means comprises a magnetic brush developing means.

10. In an electrostatic reproducing apparatus comprising a surface adapted to receive an electrostatic charge pattern, means for forming said electrostatic pattern on said surface, and means for developing said electrostatic charge pattern to render it visible, said development means including an electrically biased development electrode in close proximity to said surface, said electrode being connected to a reference potential through an impedance means so that the charge produced on the electrode during the development of said pattern provides the bias potential for said electrode, the improvement wherein, said impedance means comprises: at least one zener diode connected between said electrode and said reference potential so that the charge produced on said electrode during the development of said pattern provides a reverse bias potential across said zener diode to cause it to stabilize the bias potential on said electrode at a desired level.

11. An apparatus as in claim wherein said reference potential comprises ground potential and wherein said zener diode regulates within about i 10 percent of its nominal breakdown voltage at an electrode current of about 0.1 microampheres.

12. An apparatus as in claim 11 wherein said at least one zener diode has a breakdown potential of less than about 50 volts above the background potential on said surface.

13. An apparatus as in claim 11 wherein said zener diode has a breakdown potential of less than about 100 volts below the background potential on said surface.

14. An apparatus as in claim 11 wherein a plurality of zener diodes are selectively connected between said electrode and said ground potential to provide said bias potential.

15. An apparatus as in claim 14 wherein said zener diodes are connected in series.

16. An apparatus as in claim 11 further including means for changing the bias potential on said electrode.

17. An apparatus as in claim 16 wherein said potential changing means comprises a plurality of zener diodes or combinations of zener diodes wherein each diode or diode combination has a differing breakdown potential, and switching means for selectively connecting one or more of said diodes or diode combinations between said development electrode and said ground potential.

18. An apparatus as in claim 16 wherein said development means comprises a magnetic brush development means.

19. An apparatus as in claim 16 wherein said surface comprises a photoconductive surface, and wherein said means for forming an electrostatic charge pattern comprises means for uniformly charging said photoconductive surface and means for providing a light image of an original to impinge upon said charged surface so as to form said pattern.

20. An apparatus as in claim 19 further including means for transferring the developed image from said photoconductive surface to a sheet of final support ma- 

1. In a developing apparatus for developing electrostatic charge patterns on a surface, including an electrically biased development electrode in close proximity to said surface, said electrode being connected to a reference potential through an impedance means so that the charge produced on the electrode during the development of said pattern provides the bias potential for said electrode, the improvement wherein, said impedance means comprises: at least one zener diode connected between said electrode and said reference potential so that the charge produced on said electrode during the development of said pattern provides a reverse bias potential across said zener diode to cause it to stabilize the bias potential on said electrode at a desired level.
 2. An apparatus as in claim 1 wherein said reference potential comprises ground potential and wherein said zener diode regulates within about plus or minus 10 percent of its nominal breakdown voltage at an electrode current of about 0.1 microampheres.
 3. An apparatus as in claim 2 wherein said zener diode has a breakdown potential of less than about 50 volts above the background potential on said surface.
 4. An apparatus as in claim 2 wherein said zener diode has a breakdown potential of less than about 100 volts below the background potential on said surface.
 5. An apparatus as in claim 2 wherein a plurality of zener diodes are selectively connected between said electrode and said ground potential to provide said bias potential.
 6. An apparatus as in claim 5 wherein said zener diodes are connected in series.
 7. An apparatus as in claim 2 further including means for changing the bias potential on said electrode.
 8. An apparatus as in claim 7 wherein said potential changing means comprises a plurality of zener diodes or combinations of zener diodes wherein each diode or diode combination has a differing breakdown potential, and switching means for selectively connecting one or more of said diodes or diode combinations between said development electrode and said ground potential.
 9. An apparatus as in claim 8 wherein said developing means comprises a magnetic brush developing means.
 10. In an electrostatic reproducing apparatus comprising a surface adapted to receive an electrostatic charge pattern, means for forming said electrostatic pattern on said surface, and means for developing said electrostatic charge pattern to render it visible, said development means including an electrically biased development electrode in close proximity to said surface, said electrode being connected to a reference potential through an impedance means so that the charge produced on the electrode during the development of said pattern provides the bias potential for said electrode, the improvement wherein, said impedance means comprises: at least one zener diode connected between said electrode and said reference potential so that the charge produced on said electrode during the development of said pattern provides a reverse bias potential across said zener diode to cause it to stabilize the bias potential on said electrode at a desired level.
 11. An apparatus as in claim 10 wherein said reference potential comprises ground potential and wherein said zener diode regulates within about + or - 10 percent of its nominal breakdown voltage at an electrode current of about 0.1 microampheres.
 12. An apparatus as in claim 11 wherein said at least one zener diode has a breakdown potential of less than about 50 volts above the background potential on said surface.
 13. An apparatus as in claim 11 wherein said zener diode has a breakdown potential of less than about 100 volts below the background potential on said surface.
 14. An apparatus as in claim 11 wherein a plurality of zener diodes are selectively connected between said electrode and said ground potential to provide said bias potential.
 15. An apparatus as in claim 14 wherein said zener diodes are connected in series.
 16. An apparatus as in claim 11 further including means for changing the bias potential on said electrode.
 17. An apparatus as in claim 16 wherein said potential changing means comprises a plurality of zener diodes or combinations of zener diodes wherein each diode or diode combination has a differing breakdown potential, and switching means for selectively connecting one or more of said diodes or diode combinations between said development electrode and said ground potential.
 18. An apparatus as in claim 16 wherein said development means comprises a magnetic brush development means.
 19. An apparatus as in claim 16 wherein said surface comprises a photoconductive surface, and wherein said means for forming an electrostatic charge pattern comprises means for uniformly charging said photoconductive surface and means for providing a light image of an original to impinge upon said charged surface so as to form said pattern.
 20. An apparatus as in claim 19 further including means for transferring the developed image from said photoconductive surface to a sheet of final support material. 